CVE-2026-42857Public exploitNetwork attack vector

Email spoofing and tracking via unsanitized style tags

Published May 11, 2026 · Updated May 11, 2026

CSS injection in Open edX Platform from Sumac through releases before Ulmo allows remote authenticated users to spoof emails via discussion posts. clean_thread_html_body() omits style tags from its denylist, then Django's safe filter renders attacker-controlled CSS in discussion email templates. Course enrollment and enabled discussion emails are required; opening a message can disclose the recipient's IP address or display phishing content.

CVSS severity4.6
Medium
EPSS probability0.21%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

CSS injection in Open edX Platform from Sumac through releases before Ulmo allows remote authenticated users to spoof emails via discussion posts. clean_thread_html_body() omits style tags from its denylist, then Django's safe filter renders attacker-controlled CSS in discussion email templates. Course enrollment and enabled discussion emails are required; opening a message can disclose the recipient's IP address or display phishing content.

The record

CVE
CVE-2026-42857
Published
May 11, 2026
Updated
May 11, 2026
Vendor
Open edX
Product
Open edX Platform
Classifications
CWE-79, T1566.002
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. May 11, 2026CVE publishedPublication date reported by the CVE source.
  2. May 11, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< cddc25cd791bb78f76833896e4778f668861df12
  2. Affected versionversion=>= sumac, < ulmo

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Open edX · Open edX Platformversion=< cddc25cd791bb78f76833896e4778f668861df12; version=>= sumac, < ulmo

Published CVSS scores

4.6GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N
5.4NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1566.002Spearphishing Link

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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